1. Field of the Invention
The present invention relates to an active noise control system for controlling noise with an adaptive notch filter, and more particularly to an active noise control system which is suitable for use in a closed space such as a compartment of a mobile object having a noise source such as an engine or the like. The mobile object may be a motor vehicle such as an automobile or the like, a ship, an amphibian, a pleasure boat, a helicopter, an airplane, or the like.
2. Description of the Related Art
There have recently been proposed active noise control systems for controlling noise such as engine sounds, road noise, etc. heard in the passenger compartment of motor vehicles with control sounds radiated from speakers for reducing the noise at the ears of passengers in the passenger compartment.
It has been pointed out that when such active noise control systems fail to have an initial performance capability due to aging, the performance capability failure tends to disperse the control sound, which may possibly be output as an abnormal sound under high sound pressure from the speaker (see Japanese Patent No. 3198548 and Japanese Patent No. 3094517).
The inventors of the present application have found that even if an active noise control system operates normally (without aging), it may produce an abnormal sound under high sound pressure. Specifically, a microphone for detecting a canceling error sound representing the difference between the noise and the control sound and outputting an error signal has a sound input region, specifically, an opening defined, e.g., in a lining in the compartment of the mobile object with the microphone fixed in the lining, which may be accidentally or intentionally closed by the palm of a hand of a passenger or the like, resulting in a microphone opening closed state. When the microphone opening is closed, the gain of transfer characteristics from the speaker to the microphone is reduced, and, as a result, the control signal supplied from the adaptive notch filter to the speaker increases in level. Therefore, the control sound that is output from the speaker depending on the control signal has an unnecessarily large sound pressure, producing an abnormal sound (continuous muffled sound). The continuous muffled sound may be imagined as seashell sound that one can hear when both ears are cupped by hands or large seashells.
If the technologies of Japanese Patent No. 3198548 and Japanese Patent No. 3094517 are applied to prevent the continuous muffled sound from being produced, then control details need to be changed, e.g., updating quantities for the filter coefficients of the adaptive notch filters need to be changed or transfer functions need to be changed or convergent coefficients need to be reduced when a dispersion of the control sound is detected from the values of the filter coefficients, or the control process needs to be shut down. Therefore, when the passenger removes its hand off the microphone opening, canceling the microphone opening closed state, it is impossible to immediately perform the adaptive control process for reducing noise.